Handover control apparatus, relay router, gateway apparatus, access router, base station, mobile communication system, and handover control method

ABSTRACT

A mobile communication system  100  according to the present invention comprises handover control apparatus  1  and radio terminal unit  10 . The handover control apparatus  1  controls handover of a radio terminal unit  10  between access routers  31 - 34  provided in respective base stations  21 - 24  for radio communication with the radio terminal unit  10 . The handover control apparatus  1  counts the number of relay routers for every candidate access router  32 - 34  for a destination of the radio terminal unit  10 , from a gateway device  52  and determines a COR on the basis of the number of relay routers for every access router  32, 33, 34  thus counted.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityfrom U.S. application Ser. No. 10/379,804, filed Mar. 6, 2003, and fromprior Japanese Patent application No. 2002-060893, filed Mar. 6, 2002.The entire contents of the above application are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a handover control apparatus, a relayrouter, a gateway apparatus, an access router, a base station, a mobilecommunication system, and a handover control method.

2. Related Background Art

In recent years, packet communication with high utilization efficiencyof communication links is used for transmission and reception of databetween radio terminal units including cellular phones. In the packetcommunication, a handover involves a process of switching an accessrouter (AR; Access Router) for transmitting packets to the radioterminal unit, over to another. And there is concern that some packetsfail to arrive at the radio terminal unit during the handover (packetloss). In order to reduce the packet loss, packets transmitted from acorrespondent node with the radio terminal unit were thus bufferedduring the handover in the old access router before the handover, andthe packets were forwarded to a new access router of a switchoverdestination after completion of the handover.

SUMMARY OF THE INVENTION

The major topologies (connection forms) in the mobile communicationsystems are generally classified under the tree type and the mesh type,and in either case of these topologies, there arose the problem asdescribed below about the handover control. Namely, the packetstransmitted from the correspondent node during the handover werebuffered in the old AR. For this reason, a redundant path is madebetween the old AR and the new AR, so as to result in waste of networkresources.

There concurrently exist a communication path for forwarding packetsfrom the old AR to the new AR and a communication path for transmittingpackets from the packet source to the new AR. This causes the packetsrouted via the respective communication paths to arrive in a mixedstate, and thus raises concern about occurrence of a miss-ordering ofpackets during the handover.

The present invention has been accomplished in view of the above problemand an object of the present invention is to realize a packet forwardthrough an optimal path to a radio terminal unit after a handover,regardless of the topologies, thereby effectively utilizing the networkresources and preventing the occurrence of the miss-ordering of packetsand, as a consequence, to provide a handover control apparatus, a relayrouter, a gateway apparatus, an access router, a base station, a mobilecommunication system, and a handover control method for efficientlyperforming handover control.

In order to solve the above problem, a handover control apparatusaccording to the present invention is a handover control apparatuscomprising gateway apparatus, relay routers, and base stations havingaccess router apparatus, and adapted to control handover of a radioterminal unit between access routers provided in respective basestations for radio communication with the radio terminal unit, thehandover control apparatus comprising: counting means for counting thenumber of relay routers for every candidate access router for adestination of the radio terminal unit, from the gateway apparatus; anddetermining means for determining a COR (Cross Over Router) on the basisof the number of relay routers for every access router counted by thecounting means.

A relay router according to the present invention is a relay router fortransmission and reception of data between a plurality of base stationsor a plurality of access routers, and gateway apparatus, the relayrouter comprising: counting means for counting the number of relayrouters for every candidate access router for a destination of a radioterminal unit, from the getaway apparatus; and determining means fordetermining a COR on the basis of the number of relay routers for everyaccess router, counted by the counting means.

A gateway apparatus according to the present invention is a gatewayapparatus connected to access routers, the gateway apparatus comprising:counting means for counting the number of relay routers for everycandidate access router for a destination of a radio terminal unit, fromthe gateway apparatus; and determining means for determining a COR onthe basis of the number of relay routers for every access router countedby the counting means.

A handover control method according to the present invention is ahandover control method of controlling handover of a radio terminal unitbetween access routers provided in respective base stations for radiocommunication with the radio terminal unit, the handover control methodcomprising: a counting step wherein a handover control apparatus countsthe number of relay routers for every candidate access router for adestination of the radio terminal unit, from gateway apparatus; and adetermining step wherein the handover control apparatus determines a CORon the basis of the number of relay routers for every access routercounted in the counting step, from the gateway apparatus.

According to these aspects of the invention, counted is the number ofrelay routers for every candidate access router for the destination ofthe radio terminal unit, and the COR is determined on the basis of thenumber of relay routers for every access router, which is the result ofthe counting. Efficient handover control can be implemented by using apath with the smaller number of relay routers for the packet forward.For example, the COR is selected as a communication device with whichthe number of relay routers from the gateway apparatus via thiscommunication device to the candidate access router for the destinationof the radio terminal unit is minimum, out of communication devices(each of which is one of a gateway device, a relay router, and an accessrouter) on the route between the gateway apparatus and the accessrouter. By selecting this COR, it is feasible to perform the efficienthandover control. Namely, if packets to the radio terminal unit arebuffered in this COR during the handover, the packets can be forwardedthrough the optimal path to the radio terminal unit after the handover;it is thus feasible to effectively utilize the network resources and toprevent the occurrence of the miss-ordering of packets.

In the handover control apparatus according to the present invention,preferably, the determining means determines as the COR a relay routerwith which a sum of the numbers of relay routers for every candidateaccess router for the destination of the radio terminal unit, counted bythe counting means, is minimum, out of candidate relay routers for theCOR.

In the relay router according to the present invention, preferably, thedetermining means determines as the COR a relay router with which a sumof the numbers of relay routers for every candidate access router forthe destination of the radio terminal unit, counted by the countingmeans, is minimum, out of candidate relay routers for the COR.

In the gateway apparatus according to the present invention, preferably,the determining means determines as the COR a relay router with which asum of the numbers of relay routers for every candidate access routerfor the destination of the radio terminal unit, counted by the countingmeans, is minimum, out of candidate relay routers for the COR.

In the handover control method according to the present invention,preferably, in the determining step the handover control apparatusdetermines as the COR a relay router with which a sum of the numbers ofrelay routers for every candidate access router for the destination ofthe radio terminal unit, counted in the counting step, is minimum, outof candidate relay routers for the COR.

According to these aspects of the invention, determined as the COR is arelay router with which the sum of the numbers of relay routers forevery candidate access router for the destination of the radio terminalunit is minimum, out of the candidate relay routers for the COR.Efficient handover control can be implemented by using a path with thesmaller number of relay routers for the packet forward. According tothese aspects of the invention, a communication device with which thesum of the numbers of relay routers between the gateway apparatus andthe access routers is minimum is selected as the COR, whereby it is thusfeasible to perform the efficient handover control. Namely, if packetsto the radio terminal unit are buffered in this COR during the handover,the packets can be forwarded through the optimal path to the radioterminal unit after the handover; it is thus feasible to effectivelyutilize the network resources and to prevent the occurrence of themis-ordering of packets.

In the handover control apparatus according to the present invention,more preferably, the counting means counts the number of relay routersfor every candidate access router for the destination of the radioterminal unit in response to a COR determination request transmittedfrom the access router.

In the relay router according to the present invention, more preferably,the counting means counts the number of relay routers for everycandidate access router for the destination of the radio terminal unitin response to a COR determination request transmitted from the accessrouter.

In the gateway apparatus according to the present invention, morepreferably, the counting means counts the number of relay routers forevery candidate access router for the destination of the radio terminalunit in response to a COR determination request transmitted from theaccess router.

In the handover control method according to the present invention, morepreferably, the counting step is for the handover control apparatus tocount the number of relay routers for every candidate access router forthe destination of the radio terminal unit in response to a CORdetermination request transmitted from the access router.

According to these aspects of the invention, the number of relay routersis counted for every access router in response to the COR determinationrequest transmitted from the access router. The access router havingtransmitted the COR determination request is an old access router beforemovement of the radio terminal unit. Accordingly, for determining theCOR, the number of relay routers is counted for every access router towhich the radio terminal unit can move from the old access router, afterthe handover.

In the handover control apparatus according to the present invention,more preferably, the counting means counts the number of relay routersfrom a plurality of gateway devices to every candidate access router forthe destination of the radio terminal unit, in response to a CORdetermination request transmitted from the access router for everygateway device.

In the relay router according to the present invention, more preferably,the counting means counts the number of relay routers from a pluralityof gateway devices to every candidate access router for the destinationof the radio terminal unit, in response to a COR determination requesttransmitted from the access router for every gateway device.

In the gateway apparatus according to the present invention, morepreferably, the counting means counts the number of relay routers from aplurality of gateway devices to every candidate access router for thedestination of the radio terminal unit, in response to a CORdetermination request transmitted from the access router for everygateway device.

In the handover control method according to the present invention, morepreferably, in the counting step the handover control apparatus countsthe number of relay routers from a plurality of gateway devices to everycandidate access router for the destination of the radio terminal unit,in response to a COR determination request transmitted from the accessrouter for every gateway device.

According to these aspects of the invention, the number of relay routersfrom a plurality of gateway device to every candidate access router forthe destination of the radio terminal unit is counted in response to theCOR determination request transmitted from the access router. The numberof relay routers is counted for every gate way device. The access routerhaving transmitted the COR determination request is an old access routerbefore movement of the radio terminal unit. Therefore, for determiningthe COR, the number of relay routers for each access router to which theradio terminal unit can move from the old access router, after thehandover is counted for every gateway device. By determining the COR forevery gateway device in this way, it is feasible to determine theoptimal COR in the case where the network is constructed in such a meshtopology as to establish a plurality of paths in the mobilecommunication system. As a consequence, the optimal COR is determined,regardless of the network topologies, and it is thus feasible to performthe efficient handover control.

The handover control apparatus according to the present invention ismore preferably configured to further comprise notifying means fornotifying the access router having transmitted the COR determinationrequest, of the COR determined by the determining means.

The relay router according to the present invention is more preferablyconfigured to further comprise notifying means for notifying the accessrouter having transmitted the COR determination request, of the CORdetermined by the determining means.

The gateway apparatus according to the present invention is morepreferably configured to further comprise notifying means for notifyingthe access router having transmitted the COR determination request, ofthe COR determined by the determining means.

The handover control method according to the present invention is morepreferably configured to further comprise a notifying step of notifyingthe access router having transmitted the COR determination request, ofthe COR determined in the determining step.

According to these aspects of the invention, the access router havingtransmitted the COR determination request is notified of the COR thusdetermined. If packets to the radio terminal unit are buffered in thisCOR during the handover, the packets can be forwarded through theoptimal path to the radio terminal unit after the handover; it is thusfeasible to effectively utilize the network resources and to prevent theoccurrence of the miss-ordering of packets. As a result, the handovercontrol can be performed more efficiently.

The handover control apparatus according to the present invention ismore preferably configured to further comprise notifying means fornotifying the access router having transmitted the COR determinationrequest, of an address of the COR, determined by the determining means,corresponding to a predetermined gateway device.

The relay router according to the present invention is more preferablyconfigured to further comprise notifying means for notifying the accessrouter having transmitted the COR determination request, of an addressof the COR, determined by the determining means, corresponding to apredetermined gateway device.

The gateway apparatus according to the present invention is morepreferably configured to further comprise notifying means for notifyingthe access router having transmitted the COR determination request, ofan address of the COR, determined by the determining means,corresponding to a predetermined gateway device.

The handover control method according to the present invention is morepreferably configured to further comprise a notifying step of notifyingthe access router having transmitted the COR determination request, ofan address of the COR, determined in the determining step, correspondingto a predetermined gateway device.

According to these aspects of the invention, the access router havingtransmitted the COR determination request is notified of the CORcorresponding to the predetermined gateway device, thus determined. Ifpackets to the radio terminal unit are buffered in this COR during thehandover, the packets can be forwarded through the optimal path to theradio terminal unit after the handover; it is thus feasible toeffectively utilize the network resources and to prevent the occurrenceof the miss-ordering of packets. By notifying the access router of theCOR corresponding to the predetermined gateway device in this way, theoptimal COR can be determined, regardless of the network topologies, andthe handover control can be performed more efficiently.

The handover control apparatus according to the present invention ismore preferably configured to further comprise storage means for storingthe COR determined by the determining means and an address of a gatewaydevice corresponding to the COR, in correspondence to each other.

An access router according to the present invention is an access routerconnected to the gateway apparatus as described above and to the relayrouter as described above, the access router comprising: storage meansfor storing the COR determined by the determining means and an addressof the gateway apparatus corresponding to the COR, in correspondence toeach other.

A base station according to the present invention is a base station fordetecting handover of a radio terminal unit, the base stationcomprising: an apparatus having the function of the access router asdescribed above.

The handover control method according to the present invention is morepreferably configured to further comprise a storage step of storing theCOR determined in the determining step and an address of the gatewaydevice corresponding to the COR, in correspondence to each other instorage means.

According to these aspects of the invention, the COR thus determined isstored in correspondence to the address of the gateway devicecorresponding to the COR. When the old access router before movement byhandover refers to the COR stored and buffers packets to the radioterminal unit in this COR during the handover, the packets can beforwarded through the optimal path to the radio terminal unit after thehandover; it is thus feasible to effectively utilize the networkresources and to prevent the occurrence of the miss-ordering of packets.By storing the address of the gateway device corresponding to the COR incorrespondence thereto in this way, the optimal COR can be determined,regardless of the network topologies, and the handover control can beperformed more efficiently.

In the handover control apparatus according to the present invention,more preferably, the COR determination request is transmitted from theradio terminal unit via the access router.

In the relay router according to the present invention, more preferably,the COR determination request is transmitted from the radio terminalunit via the access router.

In the gateway apparatus according to the present invention, morepreferably, the COR determination request is transmitted from the radioterminal unit via the access router.

In the handover control method according to the present invention, morepreferably, the COR determination request is transmitted from the radioterminal unit via the access router.

The present invention is also applicable to construction and operationof a mobile communication system comprising the handover controlapparatus as described above, and a radio terminal unit, and adapted toperform radio communication between the handover control apparatus andthe radio terminal unit.

According to these aspects of the invention, the COR determinationrequest is transmitted from the radio terminal unit via the accessrouter. The radio terminal unit having transmitted the COR determinationrequest is a radio terminal unit radio-connected to the access router,before the movement by handover. Therefore, for determining the COR, thenumber of relay routers is counted for every access router to which theradio terminal unit can move after the handover.

In the handover control apparatus according to the present invention,more preferably, the COR determination request is transmitted from theaccess router.

In the relay router according to the present invention, more preferably,the COR determination request is transmitted from the access router.

In the gateway apparatus according to the present invention, morepreferably, the COR determination request is transmitted from the accessrouter.

In the handover control method according to the present invention, morepreferably, the COR determination request is transmitted from the accessrouter.

The present invention is also applicable to construction and operationof a mobile communication system comprising the handover controlapparatus as described above, and a radio terminal unit, and adapted toperform radio communication between the handover control apparatus andthe radio terminal unit.

According to these aspects of the invention, the COR determinationrequest is transmitted from the access router. The access router havingtransmitted the COR determination request is an access router to whichthe radio terminal unit before the movement by handover isradio-connected. Accordingly, for determining the COR, the number ofrelay routers is counted for every access router to which the radioterminal unit can move after the handover.

A handover control apparatus according to the present invention is ahandover control apparatus comprising gateway apparatus, relay routers,and base stations having access router apparatus, and adapted to controlhandover of a radio terminal unit between access routers provided inrespective base stations for radio communication with the radio terminalunit, the handover control apparatus comprising: counting means forcounting the number of relay routers for every candidate access routerfor a destination of the radio terminal unit, from a relay router; anddetermining means for determining a COR on the basis of the number ofrelay routers for every access router counted by the counting means.

A relay router according to the present invention is a relay router fortransmission and reception of data between a plurality of base stationsor a plurality of access routers, and gateway apparatus, the relayrouter comprising: counting means for counting the number of relayrouters for every candidate access router for a destination of the radioterminal unit, from the relay router; and determining means fordetermining a COR on the basis of the number of relay routers for everyaccess router, counted by the counting means.

A gateway apparatus according to the present invention is a gatewayapparatus connected to access routers, the gateway apparatus comprising:counting means for counting the number of relay routers for everycandidate access router for a destination of a radio terminal unit, fromthe gateway apparatus; and determining means for determining a COR onthe basis of the number of relay routers for every access router countedby the counting means.

A handover control method according to the present invention is ahandover control method of controlling handover of a radio terminal unitbetween access routers provided in respective base stations for radiocommunication with the radio terminal unit, the handover control methodcomprising: a counting step wherein a handover control apparatus countsthe number of relay routers for every candidate access router for adestination of the radio terminal unit, from a relay router; and adetermining step of determining a COR on the basis of the number ofrelay routers for every access router counted in the counting step.

According to these aspects of the invention, the number of relay routersfor every candidate access router for the destination of the radioterminal unit is counted from the relay router, and the COR isdetermined on the basis of the number of relay routers for every accessrouter, which is the result of the counting. Efficient handover controlcan be implemented by using a path with the smaller number of relayrouters for the packet forward. According to these aspects of theinvention, a communication device with the minimum sum of the numbers ofrelay routers between the relay router and the access router is selectedas the COR, and thus the efficient handover control can be performed.Namely, if packets to the radio terminal unit are buffered in this CORduring the handover, the packets can be forwarded through the optimalpath to the radio terminal unit after the handover; it is thus feasibleto effectively utilize the network resources and to prevent theoccurrence of the miss-ordering of packets.

In the handover control apparatus according to the present invention,preferably, the counting means counts the number of relay routers forevery candidate access router for the destination of the radio terminalunit in response to a COR determination request transmitted from theaccess router.

In the relay router according to the present invention, preferably, thecounting means counts the number of relay routers for every candidateaccess router for the destination of the radio terminal unit in responseto a COR determination request transmitted from the access router.

In the gateway apparatus according to the present invention, preferably,the counting means counts the number of relay routers for everycandidate access router for the destination of the radio terminal unitin response to a COR determination request transmitted from the accessrouter.

In the handover control method according to the present invention,preferably, the counting step is to count the number of relay routersfor every candidate access router for the destination of the radioterminal unit in response to a COR determination request transmittedfrom the access router.

According to these aspects of the invention, the number of relay routersis counted for every candidate access router for the destination of theradio terminal unit in response to the COR determination requesttransmitted from the access router. The access router having transmittedthe COR determination request is an old access router before themovement of the radio terminal unit. Therefore, for determining the COR,the number of relay routers is counted for every access router to whichthe radio terminal unit can move from the old access router, after thehandover.

In the handover control apparatus according to the present invention,more preferably, the COR determination request includes areas forstoring an address of a candidate relay router for the COR, an addressof every candidate access router for the destination of the radioterminal unit, the number of relay routers, and an address of the CORdetermined by the determining means.

In the relay router according to the present invention, more preferably,the COR determination request includes areas for storing an address of acandidate relay router for the COR, an address of every candidate accessrouter for the destination of the radio terminal unit, the number ofrelay routers, and an address of the COR determined by the determiningmeans.

In the gateway apparatus according to the present invention, morepreferably, the COR determination request includes areas for storing anaddress of a candidate gateway device or relay router for the COR, anaddress of every candidate access router for the destination of theradio terminal unit, the number of relay routers, and an address of theCOR determined by the determining means.

In the handover control method according to the present invention, morepreferably, the COR determination request includes areas for storing anaddress of a candidate relay router for the COR, an address of everycandidate access router for the destination of the radio terminal unit,the number of relay routers, and an address of the COR determined in thedetermining step.

The present invention is also applicable to construction and operationof a mobile communication system comprising the handover controlapparatus as described above, and a radio terminal unit, and adapted toperform radio communication between the handover control apparatus andthe radio terminal unit.

The COR determination request is successively received by the relayrouters constituting the shortest path between the old access routerbefore the movement by handover and the gateway apparatus. According tothese aspects of the invention, the relay router is allowed to refer tothe number of relay routers from a communication device (an accessrouter or a relay router) as a source of the COR determination request,in the COR determination request thus received, and to compare thenumber of relay routers with the number of relay routers from acommunication device (a relay router or a gateway device) as adestination of the COR determination request.

A handover control apparatus according to the present invention is ahandover control apparatus comprising gateway apparatus, relay routers,and base stations having access router apparatus, and adapted to controlhandover of a radio terminal unit between access routers provided inrespective base stations for radio communication with the radio terminalunit, the handover control apparatus comprising: counting means forcounting the number of relay routers for every candidate access routerfor a destination of the radio terminal unit, from a relay router; anddetermining means for determining as a COR a relay router with which asum of the numbers of relay routers for every candidate access routerfor the destination of the radio terminal unit, counted by the countingmeans, is minimum, out of candidate relay routers for the COR.

An access router according to the present invention is an access routerprovided in a base station for detecting handover of a radio terminalunit, and connected to relay routers, the access router comprising:receiving means for receiving the number of relay routers for everycandidate access router for a destination of the radio terminal unit,counted from a relay router, from the relay router; and determiningmeans for determining as a COR a relay router with which a sum of thenumbers of relay routers received by the receiving means is minimum, outof candidate relay routers for the COR.

A base station according to the present invention is a base station fordetecting handover of a radio terminal unit, the base stationcomprising: an apparatus having the function of the access router asdescribed above.

A relay router according to the present invention is a relay router fortransmission and reception of data between a plurality of base stationsor a plurality of access routers, and gateway apparatus, the relayrouter comprising: transmitting means for transmitting the number ofrelay routers for every candidate access router for a destination of aradio terminal unit, counted from a relay router, to an access router towhich the radio terminal unit was connected before handover.

A gateway apparatus according to the present invention is a gatewayapparatus connected to access routers, the gateway apparatus comprising:transmitting means for transmitting the number of relay routers forevery candidate access router for a destination of a radio terminalunit, counted from the gateway apparatus, to an old access router beforehandover.

The present invention is also applicable to construction and operationof a mobile communication system comprising the handover controlapparatus as described above, and a radio terminal unit, and adapted toperform radio communication between the handover control apparatus andthe radio terminal unit.

A handover control method according to the present invention is ahandover control method of controlling handover of a radio terminal unitbetween access routers provided in respective base stations for radiocommunication with the radio terminal unit, the handover control methodcomprising: a counting step wherein a handover control apparatus countsthe number of relay routers for every candidate access router for adestination of the radio terminal unit, from a relay router; and adetermining step of determining as a COR a relay router with which a sumof the numbers of relay routers for every candidate access router forthe destination of the radio terminal unit, counted in the countingstep, is minimum, out of candidate relay routers for the COR.

According to these aspects of the invention, determined as the COR is arelay router with which the sum of the numbers of relay routers forevery candidate access router for the destination of the radio terminalunit is minimum, out of the candidate relay routers for the COR.Efficient handover control can be implemented by using a path with thesmaller number of relay routers for the packet forward. According tothese aspects of the invention, a communication device with the minimumsum of the numbers of relay routers for every candidate access routerfor the destination of the radio terminal unit is selected as the COR,and thus the efficient handover control can be performed. Namely, ifpackets to the radio terminal unit are buffered in this COR during thehandover, the packets can be forwarded through the optimal path to theradio terminal unit after the handover; it is thus feasible toeffectively utilize the network resources and to prevent the occurrenceof the miss-ordering of packets.

The present invention will become more fully understood from thedetailed description given herein below and the accompanying drawingswhich are given by way of illustration only, and thus are not to beconsidered as limiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of the mobilecommunication system according to the present invention.

FIG. 2 is a schematic diagram for explaining the COR determining methodin the mobile communication system of the first embodiment.

FIG. 3 is a schematic diagram for explaining in more detail the CORdetermining method in the mobile communication system of the firstembodiment.

FIG. 4 is a diagram showing an example of the table in which theidentification information elements of the determined CORs and thegateway devices are stored in correspondence to each other.

FIG. 5 is a diagram showing a configuration example of data transmittedas a COR determination request in the mobile communication system of thesecond embodiment.

FIG. 6 is a diagram showing another configuration example of datatransmitted as a COR determination request in the mobile communicationsystem of the second embodiment.

FIG. 7 is a schematic diagram for explaining the COR determining methodin the mobile communication system of the second embodiment.

FIG. 8 is a diagram showing the correspondence between access routers asdestination candidates and relay routers as COR candidates.

FIG. 9 is a schematic diagram for explaining the COR determining methodin the mobile communication system of the third embodiment.

FIG. 10 (a) is a diagram showing an example of thenumber-of-relay-routers comparison table storing the numbers of relayrouters for AR0. FIG. 10 (b) is a diagram showing an example of thenumber-of-relay-routers comparison table storing the numbers of relayrouters for R2. FIG. 10 (c) is a diagram showing an example of thenumber-of-relay-routers comparison table storing the numbers of relayrouters for R1.

FIG. 11 (a) is a diagram showing an example of thenumber-of-relay-routers comparison table storing the numbers of relayrouters for GW. FIG. 11 (b) is a diagram showing an example of thenumber-of-relay-routers comparison table storing the numbers of relayrouters for GW.

FIG. 12 is a diagram showing an example of the number-of-relay-routerscomparison table in the modification of the third embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

The first embodiment of the present invention will be described below indetail with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of the mobilecommunication system in the first embodiment of the present invention.The mobile communication system 100 shown in FIG. 1 comprises handovercontrol apparatus 1 and radio terminal unit 10 such as a cellular phoneor the like. The handover control apparatus 1 is comprised of aplurality of base stations 21, 22, 23, 24 for radio communication withthe radio terminal unit 10; access routers 31, 32, 33, 34 provided inthese base stations 21 to 24; relay routers 401 to 408 directly orindirectly connected to these access routers 31 to 34; and gatewaydevices 51, 52, 53, 54.

The gateway devices 51-54, the relay routers 401-408, and the accessrouters 31-34 will be hereinafter called collectively “communicationdevices,” for convenience' sake of description. The communicationdevices constitute the mesh topology. Although not shown, thecommunication devices are wire-connected and are able to performtransmission and reception of data to and from each other. The wireconnection is not limited to only those between adjacent communicationdevices.

The mobile communication system 100 is constructed according to thetechniques of Mobile IP, Hierarchical Mobile IP, etc. proposed in IETF(Internet Engineering Task Force). The access routers 31 to 34 retaininformation about access routers (e.g., addresses thereof) to which theradio terminal unit 10 can move next. For example, the access router 32retains the addresses of the access routers 31, 33, and 34 locatedaround it, as the information about the access routers to which theradio terminal unit 10 can move.

The handover control in movement of the radio terminal unit 10 from acovered area with the access router 32 to a covered area with the accessrouters 31, 33, 34 is initiated according to signal strength of radiolinks by the radio terminal unit 10 or by the access router 32 to whichthe radio terminal unit 10 is presently connected. The handover controlapparatus 1 determines a COR (Cross Over Router) out of a plurality ofrelay routers 401 to 408 and gateway devices 51 to 54, using theaddresses of the access routers 31, 33, 34 retained in the access router32.

The COR herein is the closest to a relay router (a hierarchy close tothe radio terminal unit), out of relay routers which can forward packetsto all candidate access routers for a destination of the radio terminalunit 10 on the occasion of a handover of the radio terminal unit 10 andwith each which the number of relay routers from the gateway device tothe destination candidate access router is minimum. If packets to theradio terminal unit 10 are buffered in this COR during the handover, thepackets can be forwarded through the optimal path to the radio terminalunit after the handover, and it is thus feasible to effectively utilizethe network resources and to prevent the occurrence of the miss-orderingof packets. Accordingly, it is important to determine an appropriateCOR, in order to perform the handover control efficiently.

FIG. 2 is a schematic diagram showing a positional relation among theaccess routers, relay routers, and gateway devices constituting thehandover control apparatus according to the present invention. In FIG.2, AR1 indicates an old access router before movement by a handover, andAR2 is a candidate access router for a destination by the handover. R1,R2, R3, and GW denote relay routers or a gateway device as candidatesfor the COR. Furthermore, GW indicates that the COR to be determined isintended for communication paths between the GW and AR2.

The following will describe a determining method of the COR in the casewhere the radio terminal unit 10 undergoes a handover from the accessrouter 31 to the access router 32, with reference to FIG. 2. First, theaccess router 31 (AR1) transmits a COR determination request to thegateway device 52 (GW). This COR determination request includes at leastthe address of the access router 32 (AR2) as a candidate for thedestination by the handover, and the number of relay routers N(AR1) inthe communication path from the gateway device 52 to the access router32 (the communication route via dashed-line arrow A shown in FIG. 2) inthe case where the access router 31 is determined as the COR.

The number of relay routers herein refers to the number of relay routersvia which data of packets or the like is routed from the gateway deviceto the access router. The number of relay routers is minimum on theoccasion of transmission of data from the gateway device to the accessrouter, whereby the communication path between the gateway device andthe access router can be the shortest route.

In the preferred embodiment, the COR determination request istransmitted from AR1 through the shortest route to GW, as indicated byarrows M of solid lines in FIG. 2. On this occasion, the CORdetermination request is routed via R1, R2, and R3 located between AR1and GW. The COR determination request may also be arranged to bedirectly transmitted from AR1 to each communication device of R1, R2,R3, and GW.

When the relay router 402 (R1) then receives the COR determinationrequest from the access router 31, it counts the number of relay routersN(R1) in the communication path from the gateway device 52 to the accessrouter 32 (the communication route via dashed-line arrow B shown in FIG.2) in the case where the relay router 402 is determined as the COR.Subsequently, the relay router 402 compares N(R1) with a value obtainedby addition of 1 to the number of relay routers N(AR1).

The reason why 1 is added to the number of relay routers in the casewhere the router located in closer to the radio terminal unit is set asthe COR, on the occasion of the comparison between the numbers of relayrouters is as follows. For example, if the number of relay routers N(R1)in the case of the relay router 402 being the COR is directly comparedwith the number of relay routers N(AR1) in the case of the access router31 being the COR (without addition of 1), N(AR1) becomes smaller thanN(R1), though the number of relay routers between the relay router 402and the access router 32 is equal to that between the access router 31and the access router 32; therefore, an impartial comparison of thenumbers of relay routers cannot be made between communication devices.Thus “1,” which is a difference in the number of relay routers betweenadjacent communication devices, is added to N(AR1), whereby an impartialcomparison can be made between the numbers of relay routers from therespective communication devices to the access router as a destinationcandidate. It was supposed in the above description that the comparisonof the numbers of relay routers was made between the relay router andthe access router, but much the same method is also applied tocomparisons of the numbers of relay routers between a relay router and arelay router and between a relay router and a gateway device.

When the result of the comparison is N(AR1)+1≦N(R1), AR1 located incloser to the radio terminal unit is determined as the COR, out of AR1and R1. In contrast to it, when N(AR1)+1>N(R1), the relay router 402transmits the COR determination request to the relay router 406 locatedin farer from the radio terminal unit than the relay router 402.

This COR determination request includes at least the address of thecandidate access router 32 (AR2) for the destination by the handover,and the number of relay routers N(R1) in the communication path from thegateway device 52 to the access router 32 in the case where the relayrouter 402 is determined as the COR. The identification informationincluded in the COR determination request does not always have to belimited to the address, but it can be any information that permitsidentification of the access router. For example, it may be an ID toidentify the communication device.

Subsequently, the relay router 406 (R2), receiving the COR determinationrequest from the relay router 402, counts the number of relay routersN(R2) in the communication path from the gateway device 52 to the accessrouter 32 (the communication route via dashed-line arrow C shown in FIG.2) in the case where the relay router 406 is determined as the COR. Thenthe relay router 406 compares N(R2) with a value obtained by addition of1 to the number of relay routers N(R1).

When the result of the comparison is N(R1)+1≦N(R2), R1 located in closerto the radio terminal unit is determined as the COR, out of R1 and R2.In contrast to it, when N(R1)+1>N(R2), the relay router 406 transmitsthe COR determination request to the relay router 410 located in farerfrom the radio terminal unit than the relay router 406.

This COR determination request includes at least the address of thecandidate access router 32 (AR2) for the destination by the handover,and the number of relay routers N(R2) in the communication route fromthe gateway device 52 to the access router 32 in the case where therelay router 406 is determined as the COR. Processing similar to thatfor the relay router 406 will be carried out for the relay router 410and the gateway device 52 to count the number of relay routers.

FIG. 3 is a schematic diagram for explaining COR notification processingin the case where the result in the above COR determining method isN(R2)+1≦N(R3). When the process of counting the number of relay routersin the case of the relay router 410 being determined as the COR resultsin N(R2)+1≦N(R3), R2, i.e., the relay router 406 being the relay routerlocated in closer to the radio terminal unit is determined as the COR,out of R2 and R3. The address of the COR thus determined is transmittedfrom the relay router 410 to the access router 31, as indicated by anarrow N1 of a chain line in FIG. 3.

The address of the COR thus determined may be transmitted from thegateway device 52 to the access router 31, as indicated by an arrow N2of a chain line in FIG. 3. In this case, however, the relay router 410transmits the COR determination request including the address of the COR(relay router 406), to the gateway device 52 and the gateway device 52receives the COR determination request from the relay router 410. Thispermits the gateway device 52 to recognize the COR thus determined andto notify the access router 31 of the COR.

Eventually, comparisons of the numbers of relay routers are made aboutthe five communication paths passing the routes indicated by thedashed-line arrows A to E from the gateway device via each communicationdevice in FIG. 2 and FIG. 3. This permits the apparatus to compare thenumbers of relay routers from the gateway device 52 via each of therelay routers 410, 406, 402, and the access router 31 to the accessrouter 32, for every case where each communication device is assumed tobe the COR.

The present embodiment was described on the assumption that thecandidate access router for the destination by the handover was only onedevice of access router 32, for convenience' sake of description, butthe COR will be determined on the basis of the numbers of relay routerscounted for every access router if there exist a plurality of candidateaccess routers for the destination, e.g., the access routers 32, 33, and34.

As described above, the mobile communication system 100 in the firstembodiment comprises the handover control apparatus 1 and the radioterminal unit 10. The handover control apparatus 1 comprises the gatewaydevices 51 to 54, the relay routers 401 to 412, and the access routers31 to 34. Furthermore, the handover control apparatus 1 controls thehandover of the radio terminal unit 10 between the access routers 31 to34 provided in the respective base stations 21 to 24 for radiocommunication with the radio terminal unit 10. The handover controlapparatus 1 comprises the counting means for counting the number ofrelay routers from the gateway device 52 to the candidate access router32-34 for the destination of the radio terminal unit 10, for everyaccess router; and the determining means for determining the COR on thebasis of the number of relay routers for every access router, counted bythe counting means.

The efficient handover control can be implemented by using a path withthe smaller number of relay routers for the packet forward. For example,the efficient handover control can be performed when the communicationdevice with the minimum number of relay routers between the gatewaydevice 52 and the access router 32-34 is selected as the COR. Namely, ifpackets to the radio terminal unit 10 are buffered in this COR duringthe handover, the packets can be forwarded through the optimal path tothe radio terminal unit 10 after the handover, and it is thus feasibleto effectively utilize the network resources and to prevent theoccurrence of the miss-ordering of packets.

In the mobile communication system 100 in the present embodiment, it ispreferable to determine as the COR a relay router with which the sum ofthe numbers of relay routers counted for every access router is minimum,out of the candidate relay routers for the COR. This makes minimum thesum of the numbers of relay routers from the gateway device via the CORas a route switching point to the access routers, and thus enables moreefficient handover control.

The present embodiment described the COR determining method intended forthe communication paths from the gateway device 52 to the access routers32 to 34, but the COR determining method is also applied similarly tothat between the other gateway devices 51, 53, 54 and the access router32-34.

Namely, the access router 32 transmits the COR determination request toa plurality of gateway devices 51, 53, 54 through the paths indicated byarrows X of solid lines in FIG. 1, whereby the COR is determined for thecommunication paths from the respective gateway devices to the accessrouter. The CORs determined are transmitted along with the address ofthe corresponding gateway device through the paths indicated by arrows Yof dashed lines in FIG. 1, from the gateway devices 51, 53, 54 to theaccess router 32. This allows the access router 32 to retain the CORsfor the respective gateway devices.

FIG. 4 is a diagram showing an example of a table retained in the accessrouter 31, in which the CORs determined according to the above CORdetermining method are stored in correspondence to the identificationinformation of the gateway devices (e.g., the addresses thereof). TheCOR table 311 shown in FIG. 4 has a gateway device address storage area311 a and a COR address storage area 311 b. In FIG. 4, the addresses ofthe gateway devices and the CORs are indicated using the same numeralsas the reference symbols in the drawing.

In the gateway device address storage area 311 a, the gateway addresses(e.g., “51,” “52,” “53,” and “54”) are stored as the identificationinformation of the gateway devices being the destination of the CORdetermination request. In the COR address storage area 311 b, the routeraddresses (e.g., “405,” “406,” “403,” and “408”) are stored as theidentification information of the relay routers determined as the COR).The access router 31 can easily and quickly identify the CORs determinedfor the respective gateway devices by referring to the data stored inthe COR table 311. By determining the COR for each gateway device inthis way, it is feasible to determine the optimal COR even if the systemis constructed in the mesh topology in which a plurality ofcommunication paths arise in the mobile communication system. For thisreason, the optimal COR can be determined, regardless of the networktopologies, and the handover control can be performed efficiently.

In the present embodiment the COR determination request was transmittedfrom the old access router 32 before movement of the radio terminal unit10 by the handover, but it may be transmitted from the radio terminalunit 10 via the access router 32.

The radio terminal unit 10 having transmitted the COR determinationrequest is one connected to the access router before the movement by thehandover. Accordingly, for determining the COR, the handover controlapparatus 1 counts the number of relay routers for every access routerto which the radio terminal unit 10 can move after the handover. Even inthe case where the old access router 32 before the movement by thehandover is not provided with the COR determination requesting function,the radio terminal unit 10 located in the covered area of the accessrouter 32 can transmit the COR determination request to the gatewaydevice 52, whereby the COR determination processing can be executed.

Second Embodiment

The second embodiment of the present invention will be described belowin detail with reference to the drawings. In the first embodiment, theCOR was determined on the basis of the number of relay routers from thegateway device to the candidate access router for the destination of theradio terminal unit. In contrast to it, the present embodiment isconfigured so that the COR is determined on the basis of the number ofrelay routers from a candidate relay router or gateway device for theCOR to every candidate access router for the destination of the radioterminal unit.

The major configuration of the mobile communication system in thepresent embodiment is much the same as the configuration of the mobilecommunication system 100 described with reference to FIG. 1, and thusthe illustration and detailed description of the configuration areomitted herein while the components are denoted by the same referencesymbols. The following will detail the COR determining method in themobile communication system in the second embodiment.

FIG. 5 is a diagram showing a configuration example of data transmittedas a COR determination request in the present embodiment. The CORdetermination request 322 shown in FIG. 5 is a COR determination requesttransmitted from the access router 32 to the relay router 403, and has arelay router address storage area 322 a, an access router addressstorage area 322 b, a number-of-relay-routers storage area 322 c, and aCOR candidate address storage area 322 d. In FIG. 5, the addresses ofthe respective communication devices including the routers are denotedby the same numerals as the reference symbols in the drawing.

The relay router address storage area 322 a is a data area storing anaddress of a communication device as a principal device for counting thenumber of relay routers. For example, where the access router 32 is aprincipal device for counting, “32” is stored in the relay routeraddress storage area 322 a, as shown in FIG. 5.

The access router address storage area 322 b is a data area storingaddresses of candidate access routers for the destination of the radioterminal unit 10 by a handover. For example, where an old access routeris the access router 32, it is anticipated that the radio terminal unitmoves to one of the access routers 31, 33, and 34 during the nexthandover; thus “31,” “33,” and “34” are stored in the access routeraddress storage area 322 b, as shown in FIG. 5.

The number-of-relay-routers storage area 322 c is a data area storingthe numbers of relay routers from the communication device to the accessrouters in every case where the corresponding communication device isdetermined as a COR and where the radio terminal unit 10 moves to eachcorresponding access router. For example, where the correspondingcommunication device is the access router 32, numbers “4,” “3,” and “3”indicating the numbers of relay routers to the respective access routers31, 33, and 34 are stored in the number-of-relay-routers storage area322 c, as shown in FIG. 5.

The COR candidate address storage area 322 d is a data area storing anaddress of a relay router or an address of a gateway device as acandidate for the COR, which received the COR determination request. Theaddress is determined for every access router and is stored in the CORcandidate address storage area 322 d corresponding to each access routerat the time of determination of a candidate. Therefore, the CORcandidate address storage area 322 d is blank corresponding to theaccess routers before the determination of the COR candidate, as shownin FIG. 5.

The COR determination request having the above-stated configuration issuccessively received by the relay routers and gateway device located inthe shortest path connecting the old access router before the movementby the handover to the gateway device. Each of the relay routers andgateway device refers to the number of relay routers from the sourcecommunication device of the COR determination request and compares thenumber of relay routers with the number of relay routers from thedestination communication device of the COR determination request.

In the case of the COR determination request having the above-statedconfiguration, it is judged that the COR candidates about thecorresponding access routers are determined at the time when thecandidates for the COR corresponding to the respective access routers31, 33, and 34 are stored in the COR candidate address storage area.Accordingly, it is judged that the CORs are determined about the accessrouter 32 at the time when the COR candidates are stored in all thecells of the COR candidate address storage area included in the CORdetermination request. This permits the relay router or the gatewaydevice having counted the number of relay routers, to readily know thatthe COR determination processing is completed, by referring to the CORcandidate address storage area, without access to any othercommunication device.

The COR determination request can also be configured in the form shownin FIG. 6. As shown in FIG. 6, the COR determination request 4061 isdifferent from the aforementioned COR determination request 322 in thatthe COR determination request 4061 is provided with an additional flagstorage area 4061 e. A relay router address storage area 4061 a, anaccess router address storage area 4061 b, a number-of-relay-routersstorage area 4061 c, and a COR candidate address storage area 4061 d aremuch the same as the relay router address storage area 322 a, accessrouter address storage area 322 b, number-of-relay-routers storage area322 c, and COR candidate address storage area 322 d, respectively, asdetailed above. Therefore, the detailed description thereof is omittedherein and the flag storage area 4061 e will be detailed below.

The flag storage area 4061 e stores binary flags for indicatingcompletion of a decision on whether the corresponding communicationdevice is a COR candidate, about each corresponding access router.Namely, for the access routers 33, 34 with the corresponding flag of“1,” the decision on whether the relay router 406 is a candidate for theCOR is completed, and for the access router 31 with the flag of “0,” thedecision on whether the relay router 406 is a candidate for the COR isincomplete. The COR determination request may also be arranged toindicate completion or incompletion of the decision by deleting from theCOR determination request a data area corresponding to an access routerfor which the decision on whether the communication device is a CORcandidate is completed.

FIG. 7 is a schematic diagram showing a positional relation among theaccess routers, relay routers, and gateway devices constituting thehandover control apparatus in the present embodiment. In FIG. 7, AR0indicates an old access router before movement by a handover, and AR1,AR2, and AR3 candidate access routers for the destination by thehandover. R1 and R2 indicate candidate relay routers for the COR.Furthermore, GW indicates that the COR to be determined is intended forcommunication paths between the GW and access routers.

In FIG. 7, the addresses of the respective communication devices aredenoted by AR0, AR1, AR2, AR3, R1, R2, and GW, in order to elucidate thecorrespondence between the communication devices (access routers, relayrouters, and gateway device) and their addresses. A solid line X1 inFIG. 7 indicates the shortest path between AR0 and GW. In FIG. 7solid-line arrows Y1 indicate a transmission path of the CORdetermination request. In the same figure dashed-line arrows Y2represent the shortest paths between the communication devices locatedat the both ends of the dashed-line arrows.

The following will describe the COR determining method in the case wherethe radio terminal unit 10 is supposed to undergo a handover from theaccess router 32 to one of the access routers 31, 33, and 34, withreference to FIG. 7. Just as in the case of the first embodiment, theaccess router 32 retains the addresses of the access routers 31, 33, and34 to which the radio terminal unit 10 can move next.

First, the old access router 32 (AR0) before movement by the handovercounts the number of relay routers to each of the access routers 31, 33,34 and stores the result of the counting in the corresponding cells ofthe number-of-relay-routers storage area. In this step, the principaldevice for the counting is the access router 32, and the numbers ofrelay routers to the access router 34 (AR1), to the access router 33(AR2), and to the access router 31 (AR3) are “4,” “2,” and “2,”respectively, as apparent from FIG. 7; the COR determination requestthus goes into the state indicated by the COR determination request 323.

Then the access router 32 transmits the COR determination request 323 tothe relay router 403. The relay router 403, receiving the CORdetermination request 323, counts the number of relay routers from therelay router 403 to each of the access routers 31, 33, and 34 and storesthe result of the counting in the corresponding cells of thenumber-of-relay-routers storage area in the COR determination request.In this step, the principal device for the counting is the relay router403, and the numbers of relay routers to the access routers 31, 33, and34 are “3,” “1,” and “1,” respectively; the COR determination requestthus goes into the state indicated by the COR determination request4031.

In the next step, the relay router 403 transmits the COR determinationrequest 4031 to the relay router 406. The relay router 406, receivingthe COR determination request 4031, counts the number of relay routersfrom the relay router 406 to each of the access routers 31, 33, and 34and stores the result of the counting in the corresponding cells of thenumber-of-relay-routers storage area. In this step, the principal devicefor the counting is the relay router 406, and the numbers of relayrouters to the access routers 31, 33, and 34 are “2,” “2,” and “2,”respectively. Accordingly, “2,” “2,” and “2” are stored as numeralsindicating the numbers of relay routers in the number-of-relay-routersstorage area.

Then the relay router 403 compares the numbers of relay routers inaccordance with the COR determining method as described with referenceto FIGS. 2 and 3. Namely, it performs the process of comparing thenumber of relay routers N(R2) from the relay router 403 with the valueN(AR0)+1 obtained by addition of 1 to the number of relay routers fromthe access router 32, for every access router 31, 33, 34. As a result ofthe comparison, the relay router 403 determines AR0 located in closer tothe radio terminal unit below R2, as a COR candidate, for any accessrouter satisfying N(AR0)+1≦N(R2). Subsequently, the comparison processis carried out between the value N(R2)+1 obtained by addition of 1 tothe number of relay routers from the relay router 403, and the number ofrelay routers N(R1) from the relay router 406. As a result of thecomparison, the relay router 406 determines R2 located in closer to theradio terminal unit below R1, as a COR candidate, for any access routersatisfying N(R2)+1≦N(R1).

In the present embodiment, the numbers of relay routers corresponding tothe access router 33 (AR2) and the access router 31 (AR3) both satisfyN(R2)+1≦N(R1) in the relay router 406. Accordingly, R2, i.e., the relayrouter 403 is determined as a candidate for the COR in the case wherethe radio terminal unit 10 moves from the access router 32 to the accessrouters 33, 31. The address of the relay router 403 thus determined isstored in the corresponding cells of the COR candidate address storagearea in the COR determination request. Since at this point the COR inthe case of the radio terminal unit 10 moving to the access router 34 isnot determined yet, the cell of the COR candidate address storage areacorresponding to the access router 34 still remains blank. As a result,the COR determination request goes into the state indicated by the CORdetermination request 4061.

The gateway device 52 located at the end of the transmission path of theCOR determination request also performs much the same processing as therelay router 406 did. Namely, the gateway device 52, receiving the CORdetermination request 4061 transmitted from the relay router 406, countsthe number of relay routers from the gateway device 52 to the accessrouter 34, and stores the result of the counting in the correspondingcell of the number-of-relay-routers storage area in the CORdetermination request. In this step, the principal device for thecounting is the gateway device 52 and the numbers of relay routers tothe access routers 34 are “3,” “2,” and “2,” respectively. Accordingly,“3,” “2,” and “2” are stored as numerals indicating the numbers of relayrouters in the number-of-relay-routers storage area.

Then the gateway device 52 performs the comparison process between thevalue N(R1)+1 obtained by addition of 1 to the number of relay routersfrom the relay router 406 and the number of relay routers N(GW) from thegateway device 52, for the access router 34. As a result of thecomparison, the gateway device 52 determines R1 located in closer to theradio terminal unit below GW, as a COR, for any access router satisfyingN(R1)+1≦N(GW).

In the present embodiment, the number of relay routers corresponding tothe access router 34 (AR1) satisfies N(R1)+1≦N(GW). Accordingly, therelay router 406 is determined as a COR in the case where the radioterminal unit 10 moves from the access router 32 to the access router34. The address of the relay router 406 determined is stored in the CORcandidate address storage area in the COR determination request. As aconsequence, the COR determination request goes into the state indicatedby the COR determination request 521. FIG. 8 shows the correspondencebetween the access routers as destination candidates and the relayrouters as COR candidates, settled by the above COR determinationprocessing.

Since the COR in the case of the radio terminal unit 10 moving to theaccess router 34 was determined, the data was stored in all the cells ofthe COR candidate address storage area in the COR determination request.In other words, the relay router 406 being the COR in the movement tothe access router 34 is the last determined COR, and this COR isdetermined as a COR in the handover of the radio terminal unit 10 fromthe access router 32. The gateway device 52 notifies the access router32 of the CORs thus determined.

In the present embodiment, the COR was determined with reference to thenumber of relay routers from the candidate relay router for the COR tothe candidate access router for the destination of the radio terminalunit, but the COR may also be determined with reference to the number ofrelay routers from a candidate relay router or gateway device for theCOR to the access router as a destination of the radio terminal unit.

As described above, the mobile communication system in the secondembodiment comprises the handover control apparatus and the radioterminal unit 10. The handover control apparatus comprises the gatewaydevices 51 to 54, the relay routers 401 to 408, and the access routers31 to 34. Furthermore, the handover control apparatus controls thehandover of the radio terminal unit 10 between the access routers 31-34provided in the respective base stations for radio communication withthe radio terminal unit 10. The handover control apparatus comprises thecounting means for counting the number of relay routers from the relayrouter 403, 406 to the candidate access router 31, 33, 34 for thedestination of the radio terminal unit 10, for every access router onthe occasion of the handover of the radio terminal unit 10; and thedetermining means for determining the COR on the basis of the number ofrelay routers for every access router counted by the counting means.

The efficient handover control can be implemented by using a path withthe smaller number of relay routers for the packet forward. When thecommunication device with the minimum number of relay routers betweenthe relay router 403, 406 and the access router 31, 33, 34 is selectedas the COR, the efficient handover control can be performed. Namely, ifpackets to the radio terminal unit 10 are buffered in this COR duringthe handover, the packets can be forwarded through the optimal path tothe radio terminal unit 10 after the handover; it is thus feasible toeffectively utilize the network resources and to prevent the occurrenceof the miss-ordering of packets.

In the mobile communication system in the second embodiment, the gatewayaddress is also stored as identification information of the gatewaydevice as a destination of the COR determination request in the gatewaydevice address storage area, as in the first embodiment. The routeraddresses are stored as identification information of the relay routersdetermined as CORs, in the COR address storage area. The access router32 is allowed to easily and quickly identify the COR determined forevery gateway device, by referring to the data stored in the COR table.When the present embodiment is also configured to determine the COR forevery gateway device, the optimal COR can be determined even if thesystem is configured in the mesh topology in which a plurality ofcommunication routes arise in the mobile communication system.Therefore, the optimal COR can be determined, regardless of the networktopologies, and the handover control can be performed efficiently.

Third Embodiment

The third embodiment of the present invention will be described below indetail with reference to the drawings. In the second embodiment, therelay router or the gateway device was used to determine the CORcandidate and COR. In contrast to it, the access router is used todetermine the COR candidate and COR in the present embodiment.

The principal configuration of the mobile communication system in thepresent embodiment is much the same as the configuration of the mobilecommunication system 100 described with reference to FIG. 1, and thusthe illustration and detailed description of the configuration areomitted herein while the same reference symbols denote the components.The following will detail the COR determining method in the mobilecommunication system in the third embodiment.

FIG. 9 is a schematic diagram showing a positional relation among theaccess routers, relay routers, and gateway devices constituting thehandover control apparatus in the present embodiment. In FIG. 9, AR0designates an old access router before movement by a handover, and AR1,AR2, and AR3 candidate access routers for the destination by thehandover. R1 and R2 represent candidate relay routers for the COR, whichreceived the COR determination request. Furthermore, GW indicates thatthe COR to be determined is intended for communication paths between theGW and access routers.

In FIG. 9, the addresses of the respective communication devices aredenoted by AR0, AR1, AR2, AR3, R1, R2, and GW, in order to elucidate thecorrespondence between the communication devices (access routers, relayrouters, and gateway device) and their addresses. In FIG. 9 a solid lineX2 indicates the shortest path between AR0 and GW. In FIG. 9 arrows Y3of solid lines represent a transmission path of the COR determinationrequest. In the same figure dashed-line arrows Y4 indicate the shortestpaths between the communication devices located at the both ends of thedashed-line arrows.

The following will describe the COR determining method in the case wherethe radio terminal unit 10 is supposed to undergo a handover from theaccess router 32 to one of the access routers 31, 33, and 34, withreference to FIG. 9. Just as in the case of the second embodiment, theaccess router 32 retains the addresses of the access routers 31, 33, 34to which the radio terminal unit 10 can move next.

First, the access router 32 generates a COR determination request 324.The COR determination request 324 is generated by much the same methodas the COR determination request 323 in the second embodiment was.Subsequently, the access router 32 transmits the COR determinationrequest 324 to the relay router 403.

The access router 32 stores the numbers of relay routers (3, 2, 2) fromthe access router 32 (AR0) to the access routers 31, 33, 34 (AR3, AR2,AR1), counted in the generation of the COR determination request 324, ina number-of-relay-routers storage area 325 a of anumber-of-relay-routers comparison table 325 shown in FIG. 10 (a). As aconsequence, the number-of-relay-routers comparison table 325 goes intothe state shown in FIG. 10 (a).

The relay router 403, receiving the COR determination request 324 fromthe access router 32, generates a COR determination request 4032 by muchthe same method as that for the COR determination request 4031 in thesecond embodiment. The relay router 403 notifies the access router 32 ofthe numbers of relay routers from the relay router 403 (R2) to theaccess routers 31, 33, 34 (AR3, AR2, AR1), counted in the generation ofthe COR determination request 4032.

The access router 32, receiving the notification, adds 1 to each of thenumbers of relay routers (3, 2, 2) corresponding to the access router 32(AR0) in order to compare the numbers of relay routers by much the samemethod as the comparison process of the numbers of relay routers in thesecond embodiment (i.e., the comparison method described with referenceto FIGS. 2 and 3). As a consequence, the number-of-relay-routers storagearea 325 a is updated, and “4, 3, 3” are stored therein.

Then the access router 32, receiving the numbers of relay routers fromthe relay router 403, stores the numbers of relay routers (2, 1, 1) in anumber-of-relay-routers storage area 325 b of thenumber-of-relay-routers comparison table 325. As a consequence, thenumber-of-relay-routers comparison table 325 goes into the state shownin FIG. 10 (b).

The relay router 406 and the gateway device 52 perform much the sameprocessing as the relay router 403 did. Namely, the relay router 406notifies the access router 32 of the numbers of relay routers from therelay router 406 (R1) to the access routers 31, 33, 34 (AR3, AR2, AR1).The numbers of relay routers thus notified of are stored in anumber-of-relay-routers storage area 325 c of thenumber-of-relay-routers comparison table 325. As a consequence, thenumber-of-relay-routers comparison table 325 goes into the state shownin FIG. 10 (c).

Furthermore, the gateway device 52 notifies the access router 32 of thenumbers of relay routers from the gateway device 52 (GW) to the accessrouters 31, 33, 34 (AR3, AR2, AR1). The numbers of relay routers thusnotified of are stored in the number-of-relay-routers storage area 325 dof the number-of-relay-routers comparison table 325. As a consequence,the number-of-relay-routers comparison table 325 goes into the stateshown in FIG. 11 (a).

After completion of the storage and update for all the numbers of relayrouters corresponding to AR0, R2, R1, and GW, the access router 32calculates a sum of the numbers of relay routers for every communicationdevice. The sums thus calculated are stored as the total numbers ofrelay routers in a number-of-relay-routers storage area 325 e of thenumber-of-relay-routers comparison table 325. As a consequence, thenumber-of-relay-routers comparison table 325 goes into the state shownin FIG. 11 (b).

Subsequently, the access router 32 compares the sums of relay routersstored each in the number-of-relay-routers storage area 325 e, todetermine a communication device with the smallest sum as the COR out ofAR0, R2, R1, and GW. In the present embodiment, the smallest sum is “8.”There exist two communication devices corresponding to the pertinentsum, R1 and GW. In such a case, R1 closer to the access router 32, i.e.,the relay router 406 is determined as a COR, in order to make thecommunication path shorter from the COR to the access router.

As described above, the mobile communication system in the thirdembodiment comprises the handover control apparatus and the radioterminal unit 10. The handover control apparatus comprises the gatewaydevices 51 to 54, the relay routers 401 to 408, and the access routers31 to 34. Furthermore, the handover control apparatus controls thehandover of the radio terminal unit 10 between the access routers 31-34provided in the respective base stations for radio communication withthe radio terminal unit 10. The handover control apparatus comprises thecounting means for counting the number of relay routers for everycandidate access router for the destination of the radio terminal unit10, from the relay router, on the occasion of the handover of the radioterminal unit 10; and the determining means for determining the relayrouter with the minimum sum of the numbers of relay routers for everyaccess router, counted by the counting means, as a COR.

Accordingly, the relay router with the minimum sum of the numbers ofrelay routers for every access router is determined as a COR, out of thecandidate relay routers for the COR. The efficient handover control canbe performed by using a path with the smaller number of relay routersfor the packet forward. Since the communication device with the minimumsum of the numbers of relay routers between the access router 32 and theaccess routers 31, 33, 34 is selected as a COR, the number of relayrouters to the destination access router becomes minimum on average.Accordingly, the efficient handover control can be performed. Namely, ifpackets to the radio terminal unit 10 are buffered in this COR duringthe handover, the packets can be forwarded through the optimal path tothe radio terminal unit 10 after the handover; it is thus feasible toeffectively utilize the network resources and to prevent the occurrenceof the miss-ordering of packets.

In the mobile communication system in the third embodiment, the gatewayaddress is stored as identification information of the gateway device asa destination of the COR determination request in the gateway deviceaddress storage area, as in the first and second embodiments. The routeraddresses are stored as identification information of the relay routersdetermined as CORs in the COR address storage area. The access router 32is allowed to easily and quickly identify the COR determined for everygateway device, by referring to the data stored in the COR table. Whenthe present embodiment is also configured to determine the COR for everygateway device, the optimal COR can be determined even if the system isconfigured in the mesh topology in which a plurality of communicationpaths arise in the mobile communication system. For this reason, theoptimal COR can be determined, regardless of the network topologies, andthe handover control can be performed efficiently.

In the present embodiment, the relay router with the minimum sum of thenumbers of relay routers for every access router was determined as aCOR, but it is also possible to configure the mobile communicationsystem according to the present invention in a modification describedbelow.

For example, the access router 32 acquires a communication device (AR0,R2, R1, GW) with the minimum number of relay routers for every accessrouter (AR1, AR2, AR3) and determines a communication device with thelargest number of times that the number of relay routers becomesminimum, as a COR.

The modification of the third embodiment will be described below indetail with reference to FIG. 12. FIG. 12 is a diagram showing anexample of the number-of-relay-routers comparison table in themodification. Each of the numbers of relay routers in circles in FIG. 12indicates that the number of relay routers is minimum among the numbersof relay routers in each common address of an access router (AR1, AR2,AR3).

In FIG. 12, with focus on the case where the destination access routeris AR1, the device with the minimum number of relay routers is R1, i.e.,the relay router 406. Similarly, with focus on the case where thedestination access router is AR2, the device with the minimum number ofrelay routers is R2, i.e., the relay router 403. Furthermore, with focuson the case where the destination access router is AR3, the device withthe minimum number of relay routers is R2, i.e., the relay router 403.Accordingly, the relay router 403, with which the number of times thatthe number of relay routers is minimum is the largest, two, isdetermined as a COR. If there appear a plurality of devices with whichthe number of times that the number of relay routers is minimum is thelargest, a device located in closer to the radio terminal unit will bedetermined as a COR, in order to make the communication path shorterfrom the COR to the access router.

In the modification, a redundant path can be made between the COR andthe access router, but the greater number of access routers can have theminimum number of relay routers from the COR to the destination accessrouter. Therefore, the probability of the transmission path from the CORto the destination access router becoming the shortest path is high.

Furthermore, it is also possible to employ a method of obtaining thedifference in the number of relay routers from the gateway device 52 tothe access router 31, 33, 34 among the communication devices (AR0, R2,R1, GW) and determining a communication device with the maximumdifference as a COR. This permits the communication device to beselected as a COR while largely reducing redundant paths.

The COR determination processing in all the above embodiments ispreferably executed on the occasion of change in the network topology,or at intervals of a fixed time, or immediately before a handover.

In the third embodiment, the radio terminal unit 10 may be configured toexecute the sequential processing executed for the determination of theCOR by the access router 32.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

1. A relay router for transmission and reception of data between aplurality of base stations or a plurality of access routers, and gatewayapparatus, the relay router comprising: transmitting means fortransmitting the number of relay routers for every candidate accessrouter for a destination of a radio terminal unit, counted from therelay router, to an old access router before handover.
 2. A gatewayapparatus connected to access routers, the gateway apparatus comprising:transmitting means for transmitting the number of relay routers forevery candidate access router for a destination of a radio terminalunit, counted from the gateway apparatus, to an old access router beforehandover.